Hypsilophodontids
If you don't like 'em, stay away from south eastern
Australia!

The vast majority of dinosaur remains found in the south east of the
Australian continent are from hypsilophodontid dinosaurs. These small
ornithopod ("bird foot") herbivores were extremely successful,
being one of the most long-lived and wide-spread ornithopod families.
They lived from the Middle Jurassic to the Late Cretaceous (a span of about
110 million years) and have been found on every continent.

The name hypsilophodontid means "high crested tooth", refering to the
leaf shaped teeth and the long ridges that ran along their length.
The teeth
of hypsilophodontid dinosaurs, like those of most dinosaurs,
were constantly shed as they wore down, and were replaced by new teeth
waiting below them. The image to the left
shows a nearly complete dentary (lower jaw bone) from near
Inverloch in southern Victoria,
and is the type specimen of
Qantassaurus. Part of the
outer fossil bone has been removed (down to the obvious line)
to reveal the unerupted teeth below the jaw line
(the teeth are outlined in red). The four teeth seen
in the lowest row would have originally been completely inside
the jaw bone. As the teeth above them wore out, the new teeth
below them pushed them out, eventually replacing them.
The teeth were replaced one at a time as was
required and not as a complete set. Although hypsilophodontids had
cheek-teeth further back in their mouths, most species had a toothless
beak at the front of the mouth for snipping off tough vegetation.

The femur (upper leg bone) is one of the most diagnostic bones of the
hypsilophodontid skeleton, which is probably just as well since femora
(the plural of femur) comprise much of the Victorian hypsilophodontid
material.
This image
shows an isolated hypsilophodontid femur, from the
Dinosaur Cove site in Victoria. The "hanging"
fourth trochanter (red arrow) is a feature common to most hypsilophodontids,
and one that they shared with more primitive relatives such as
Lesothosaurus and Heterodontosaurus. The fourth trochanter
acted as a site for the attachment of the leg muscles.

Other features that are characteristic of hypsilophodontids are elongated
shins, four-toed feet and five-fingered hands. With shins longer than
their thigh bones hypsilophodontids would have been among the fastest of
dinosaurs. Their long tails, stiffened by tendons, would have helped them
to swerve and change direction quickly while running. Sharp claws on their
hind feet would have given them excellant traction. There would have
been few predatory dinosaurs that could have run down a hypsilophodontid.
In behaviour they have been compared to gazelles - fast running browsers
built for speed rather than for fighting off attackers.

At least five (and may be more) species of hypsilophodontid are known
from the Victorian sites. These include
Leaellynasaura amicagraphica,
Atlascopcosaurus loadsi, Fulgarotherium australe,
Qantassaurus intrepidus, and some as-yet unnamed varieties. Nowhere else in
the world is such a variety of hypsilophodontid species known from
about the same time period (in this case the Aptian/Albian, 106-115
million years ago).

One of the best preserved hypsilophodontid skeletons comes from
Dinosaur Cove. It is an articulated partial skeleton consisting of
most of the back half of the animal, including the hind legs, pelvis,
and some of the tail. It was found at the Slippery Rock site in 1989, one
of the three individual excavations at Dinosaur Cove, and was discovered
only two metres away from the type specimen of Leaellynasaura.
This may represent another specimen of the same species.
The most striking feature of this animal is the difference between
the tibiae (lower leg bones). One of them had become infected while the
animal was alive, resulting in an advanced case of osteomyelitis.
The right (healthy) tibia was 185 mm long and up to 30x10 mm wide.
However, the infected bone was only 158 mm long, with a width of
40x30 mm. The amount of extra bone growth around the shaft,
and the fact that the healthy bone in the other leg continued to grow
another 27 mm, indicates that the animal lived for several years
with the condition, most likely in constant agony. An abscess 41x16 mm
wide and 8 mm deep had formed on the bone, and probably drained pus
from an open wound in the skin.

Amazingly the infection itself may not
have been what killed the animal, at least not directly. It probably died
of complications brought on by malnutrition, since it
would not have been able to walk quickly or far enough to
find adequate food. The front half of the animal is missing, so there
is the possibility that the animal was caught by a predator.
However, most of the meat-bearing parts of the body would
have been around the hind quarters, and there is no indication
of predator-induced damage to the bones in this area (such as teeth
marks). If this animal managed to live for several years without
being able to run, and was not even scavenged when it finally died,
then this may suggest how few predators were about in the
polar forests, and perhaps a reason why the smaller and
more defenceless hypsilophodontids prefered colder conditions.
Certainly the number of juvenile hypsilophodontid bones from south eastern
Australia indicates that the animals were capable of breeding there,
so perhaps the lack of predators made this area such an attractive
place to raise young - or to avoid being eaten.

During the Early Cretaceous southeastern Australia was within the
Antarctic circle.
The average annual temperature was probably somewhere between -6° and 3° celsius
(21-37 fahrenheit). The winters were almost certainly frozen,
with up to three months of darkness punctuated only by a moon that
would have been visible for up to two weeks at a time.
It has been suggested that hypsilophodontids may have been
pre-adapted to living in colder climates. Studies of the way in
which their bones grow have shown that they remained active all
year round, whereas most other types of dinosaur show signs of
seasonal stress that affected the way their bones grew (possibly due
to being less active during winter months). One of the Dinosaur
Cove hypsilophodonts (the eternally cute Leaellynasaura)
has had a natural cast of the original brain
preserved. As well as having large eye sockets, this individual also
had enlarged optic lobes, the parts of the brain that process
vision. Improved vision may indicate that these animals remained
active during the long dark winters, whereas other species of
dinosaur may have migrated or hibernated, or at least become less active than
at warmer times of the year.

The number of
juvenile and diseased hypsilophodontid specimens from the southern
Victorian sites may indicate that these deposits were layed down by
spring melt waters, washing any creatures that did not survive the
winter (such as the very young or the sick) into river deposits and
covering them quickly. There are few large dinosaurs
represented in these fossil layers. These deposits were layed down by
river and stream channels, which would have been moving fast enough to
collect and redeposit small bones, but probably not of enough force
to do the same for extremely large dinosaur bones. Large dinosaurs
are only known from these deposits by the smallest bones in their
bodies, and even these are rare when compared to the small herbivore
remains.